1. Field of the Invention
This invention relates to a seam fabric for a machine for producing web material, in particular paper or paperboard.
2. Description of the Related Art
Seam fabrics for machines for producing web material, generally referred to also as needle felts, are often constructed with a basic structure formed as seam cloth. Such seam cloths are manufactured in an endless weaving process such that the weft threads of the seam cloth running in the belt longitudinal direction form seam loops at the two belt longitudinal ends. Said loops are joined together during the weaving operation by a seam thread or seam wire such that during the weaving operation the woven basic structure or the seam cloth actually has an endless configuration. For installation in a paper machine of the like, the two longitudinal ends are separated such that the belt can be pulled in its direction of movement into the machine, after which the two belt longitudinal ends can then be joined together again.
A tie pattern or an approach for weaving such a seam cloth as the basic structure for a seam fabric is presented in FIGS. 1 and 2. In FIG. 1 the lines 1 to 16 stand for the various weft insertions “S” running in the belt longitudinal direction while the columns 1 to 18 stand for the warp threads “K” running in the belt transverse direction. Shown is a weft/warp repeat, whereby the dark fields indicate warp elevations which form in each case during shedding. Shown in the upper part of FIG. 2 is the time-related run of the weft thread and weft insertions during the manufacturing of such a seam cloth. Also evident are two warp threads 10 and 12, of which the warp thread 10 forms the so-called lead thread and the warp thread 12 the so-called seam thread or seam wire. Between these two warp threads 10 and 12 there then lie the warp threads to be integrated in the weave, which are not shown any further. With this approach seam loops 14, 16 are formed in each case at the seam threads 12, namely in that for example first the weft 1 is inserted and forms the loop 14 over the seam thread 12 and then is returned as weft 2, namely through underneath the seam thread 12. The weft 2 then runs to the lead thread 10, is passed around it and then runs as weft 3 above the lead thread 10 always in the direction toward the other end of the weave in the weft direction. To form the seam loop 16 it is then passed first outside and then underneath around the seam thread 12 and moved back as weft 4 in the other direction toward the lead thread 10. In this way, two seam loops 14, 16 following one after the other in warp direction are formed. To form more seam loops this process is repeated in similar manner with a fifth weft whose run corresponds essentially to that of the first weft presented. The lower representations in FIG. 2 show in each case the height assignment of the two warp threads 10 and 12 in relation to the various weft insertions 1, 2, 3 and 4.
When such a seam cloth is completed and the seam thread 12 pulled out of the numerous seam loops 14 and 16 following one after the other in the warp direction, then a cloth with two longitudinal ends is obtained from the seam cloth which during the weaving operation was still an endlessly constructed seam cloth. This cloth includes two cloth sections 18, 20 between the longitudinal ends lying in the region of the seam loops 14 and 16, whereby the cloth section 18 is formed essentially with the wefts 1 and 2 and respectively following wefts, while the cloth section 20 is formed essentially with the wefts 3 and 4 and respectively following wefts. A cloth is thus provided in which the weft threads form two weft thread layers, whereby one of these weft thread layers includes essentially the wefts 1 and 4 and respective wefts, while another of these weft thread layers includes the wefts 2 and 3 and respective wefts. It is evident, therefore, that although in FIG. 2 the weft insertions with the numbers 1, 2, 3 and 4 follow one after the other in time and it is also possible as such for these weft insertions to be interpreted as respective weft threads during the weaving operation, when the seam thread 12 on the completed seam cloth is removed and the two longitudinal ends with the seam loops 14 and 16 respectively provided there are moved apart in order to create an elongated flat seam cloth, then the weft threads 1 and 4 and similarly the weft threads 2 and 3 actually continue each other and thus form in the completed and elongated flat cloth respectively a single weft thread provided with the two sections 1 and 4 and 2 and 3 respectively. Hence where talk in the following is of the weft threads 1, 2, 3 or 4, it refers respectively to, for example, the weft thread section 2 of the weft thread actually formed on the whole in the completed cloth by the weft insertions or weft thread sections 2 and 3.
FIGS. 3 and 4 show a cross-section of a seam fabric 22 constructed with such a seam cloth as the basic structure. For example, FIG. 3 shows a cross-section in the region of the cloth section 18, while FIG. 4 shows a cross-section in the region of the cloth section 20. From FIG. 3 it is evident first that such a constructed seam fabric 22 has fiber material 26 and 28 (represented only symbolically), for example felt material or fleece material, on the two sides of the basic structure 24, meaning the seam cloth. Also evident are the two layers 30, 32 of the weft threads 1, 2, whereby said weft thread layers 30, 32 form weft thread pairs 34 in which weft threads 1 and 2 are positioned essentially one above the other. It is also evident from FIG. 3 that, due to the time-related succession of the weft insertions, the wefts 1 to 2 of the respective weft thread pairs 34 do not lie exactly one above the other, which would be desirable, but have a lateral offset to each other, namely an offset in warp direction or in the belt transverse direction.
A corresponding offset also exists in the cloth section 20, which is shown in FIG. 4. On account of the manufacturing process, however, the offset between the wefts or weft thread sections 4 and 3 forming the weft thread layers 30 and 32 is inverse to the offset direction in the cloth section 18. Between these two cloth sections 18 and 20, namely where in the weaving process the weft threads are wound around the lead thread 10 and where the longitudinal ends, meaning the seam loops 14 and 16 are formed, there are in each of the weft thread pairs 34 crossover points in which the change of offset takes place.
When such a seam fabric 22, after being installed in a paper machine or the like, moves through press sections or a press nip, then because of the forces (indicated by arrows) acting on the surfaces of the seam fabric 22 the weft threads 1, 2 and 3, 4 of the respective weft thread pairs 34 are shifted more intensively in relation to each other, with the result that the entire weft thread layers 30 and 32 together with the respectively adjacent layers 26 and 28 shift sideways from the fiber material in relation to each other, meaning in the belt transverse direction. This results in particular in the thickness of the seam cloth or the basic structure being reduced slightly. Only where the individual weft threads of the weft thread pairs 34 cross over each other is the thickness somewhat greater. As previously explained, said crossover points exist at defined positions of the belt looking in the belt longitudinal direction, hence essentially all these crossover points of the various weft thread pairs run through a press nip simultaneously. As the result, the devices exerting pressure on the seam fabric 22, meaning for example the press rollers, are pushed slightly apart, whereby the pushing apart occurs twice on each complete belt pass. This leads to a so-called roller oscillation, which has a negative effect on the performance of a paper production operation and also leads in particular to an impairment of the quality of the produced web material, meaning paper.
What is needed in the art is a seam fabric for a machine for producing web material, with which the occurrence of roller oscillations can be avoided.